1. Field of the Invention
The present invention relates to a semiconductor flip-chip package. More particularly, the present invention relates to an under bump metal (UBM) for implementing a fine pitch solder ball, and a flip-chip packaging method using the same.
2. Description of the Related Art
As the speed and integration of semiconductor chips has been improved, semiconductor chip elements have become finer, and the number of I/Os has increased. Methods for packaging a semiconductor chip in a minimum space, such as a ball grid array package and a chip scale package, have recently been introduced. The semiconductor chip is packaged using diverse electric connections such as wire bonding, tape automated bonding (TAB), and flip-chip bonding. Flip-chip bonding is most effective for a high-speed, intelligent and high-density package, in which an electrode arranged on the semiconductor chip and a wafer connection terminal are directly connected.
FIGS. 1A and 1B illustrate a package using conventional flip-chip bonding. Referring to FIG. 1A, solder bumps 31 and 32, as a connection medium, are deposited on a connection terminal (not shown) on a wafer 10. The solder bumps 31 and 32 take on the shape of a sphere by a reflow process, and a semiconductor chip pad 40 is bonded thereto to assume the shape 31a, 32a. The solder bumps 31 and 32 are hereinafter referred to as solder balls 31 and 32.
To enable the solder balls 31 and 32 to easily adhere to the semiconductor chip pad 40, under bump metal (UBM) 21, 22, 41 and 42 is formed on a bonding area between the wafer 10 and the semiconductor chip pad 40. The UBM 21, 22, 41 and 42 is formed by evaporation-depositing or etching a metal such as Cr, Au, Ti and Cu, so as to enhance wettability of the solder balls 31 and 32. The UBM also prevents diffusion of the solder so that the solder component cannot penetrate into the semiconductor chip.
As electronic products are miniaturized, more attention is being paid to package size. Therefore, fine pitch solder bumps have been developed. However, in forming fine pitch solder bumps, problems often occur in reliability of the package. Especially when the semiconductor chip pad 40 is bonded onto the solder balls 31 and 32, a shear stress is applied to the solder balls 31 and 32 by pressure from the wafer 10 and the pad 40, as shown in FIG. 2B. The pitch between solder balls 31 and 32 is determined according to the size of the solder balls, temperature, and force applied to the solder balls. In the prior art, the solder balls 31 and 32 are transversely deformed by the shear stress, thereby limiting the pitch between solder balls. As a result, a fine pitch pattern is hard to achieve.